Systems for improving driving stability in vehicles, especially commercial vehicles (e.g., tractors or trailers), are known. Examples include anti-lock braking systems (“ABS”, see e.g., DE 19747689 B4), electronic braking systems (“EBS”, see e.g., DE 19854788 A1), electronic stability control (“ESC”) systems (see e.g., DE 10338879 A1) and rollover stability systems (“RSS”, see e.g., DE 10135020 A1). For all of these systems, it is important to be able to determine both the speeds of the individual wheels and the speed of the vehicle as precisely as possible.
Wheel sensors are typically used to measure wheel speeds. These comprise fixed sensors that are sensitive to magnetic fields. The sensors sense toothed rings that revolve with the wheels. Each toothed ring usually has 100 or 80 teeth. The values of wheel revolutions per minute (“rpm”) measured in this way are transmitted to the electronic control units of the various vehicle stability systems noted above and further processed therein.
Vehicle speed is calculated from the individual wheel speeds in accordance with certain known equations. Thus, vehicle speed depends on the size or circumference of the mounted tires. Knowledge of the tire size is therefore necessary for the most accurate determination of vehicle speed.
During vehicle manufacture, the appropriate tires are fitted and, at the end of the assembly line, their size is communicated to the respective built-in electronic control units or is parameterized. After the vehicle has been operated for some time, however, the values for the tires may change. For example, tires having diameters which differ from the diameters of the original tires may be installed. Furthermore, the diameters of the original tires may be reduced by wear. Consequently, the original parameterized tire sizes are no longer applicable, and an exact calculation of the vehicle speed is no longer possible. For example, worn tires will result in an erroneous increase in calculated vehicle speed since worn tires revolve at higher rates.
For individual tires with wheel speeds that deviate significantly from the calculated vehicle speed, an rpm correction (“tire compensation”) factor can be applied by the electronic control units of the various vehicle stability systems to ensure that the tire speeds as processed in the electronic control units match the speeds of the other tires. Nevertheless, this conventional compensation approach is problematic when all tire sizes deviate equally from the value originally parameterized by the manufacturer. In such case, vehicle speed may be incorrectly calculated by the electronic control units, and the correct functioning of the vehicle stability systems relying on the vehicle speed calculation cannot be assured.
Knowledge of the precise vehicle speed is particularly important for RSS, which is intended to prevent vehicle rollover on curves. With RSS, lateral acceleration of the vehicle is determined while it is traveling on a curve, and, if this exceeds a limit value, the vehicle is automatically braked. Lateral acceleration can either be measured by a separate lateral acceleration sensor or calculated from the difference of the values of wheel rpm of the right and left sides of the vehicle. A deviation between parameterized tire size and actual tire size can lead to erroneous calculation of wheel speeds. Such errors are raised to the power of two in the calculation of vehicle lateral acceleration.
If, as mentioned above, vehicle speed is overestimated because of worn tires, the vehicle may be braked too early. Thus, full advantage of the limit for rollover prevention is not taken, since the vehicle is actually traveling more slowly than assumed by the electronic control unit of the vehicle's stability system. If, on the other hand, vehicle speed is underestimated by the electronic control unit, the vehicle may be braked too late to prevent rollover.